1. Field of the Invention
The present invention relates to a fuel injector for an internal combustion engine such as a diesel engine.
2. Prior Art
Most conventional fuel injectors comprise an automatically operating valve including a needle valve pressed against a valve seat by a spring. The fuel pressure acts upon an intermediate tapered shoulder of the needle valve, so that, when the spring pressure is overcome, the needle valve is raised some distance above the valve seat and the fuel is injected into the combustion chamber of an internal combustion engine through an aperture thus formed. In such an automatically operating valve, the valve-opening pressure is always constant not only at low engine speeds but also at high engine speeds. Therefore, it is difficult to effect injection at a low rate when the engine is idling or operates for a relatively long period or effect injection at a high rate when the engine is in a high load range during which the fuel is injected for a relatively short period.
With the foregoing difficulty in view, the present assignee has proposed a two-stage fuel injector comprising a valve operative at two different valve-opening pressures. This injector houses two springs in its nozzle holder, one of which counteracts to the fuel pressure to limit the lift of a needle valve at the initial stage of fuel injector followed by the main injection occurring when the fuel pressure overcomes a combined force of the two springs.
In the two-stage fuel injector shown in Japanese Utility Model Laid-open Publication No. 57-186657, two pairs of springs and associated movable spring seats are disposed in series to act in such a manner that at the initial injection stage, the first spring seat is lifted by a distance .DELTA.1 against the force of the first spring resting on the first spring seat, and at the end of the initial inspection stage, the first spring seat is further lifted to raise the second spring seat by a distance 1-.DELTA.1 against the force of the second spring resting on the second spring seat. With this arrangement, when the engine is idling or operates under low load condition, inspection is effected at two stages (initial and full injections) to provide a low inspection rate, thereby stabilizing low speed engine operation. The two stage engine fuel injection does not take place when the engine operates at high speeds and under heavy loads because the valve-opening pressure and therefore the combined force of the first and second springs is constant. Thus, the two-stage fuel injector is still not satisfactory in that an instant, high pressure fuel injection is difficult to achieve when the engine operates at high speeds and under heavy loads.